I've been a Jeep owner for over 20 years, but this is my first time posting on a forum looking for mechanical answers, so any advice you guys can give would be greatly appreciated.

Back in December of 2013 I bought a 2002 Grand Cherokee Laredo. From the first week I got a P0300, 303 & 304. I brought it back repeatedly to the dealer who made several attempts to fix it, to no avail. Eventually the warranty ran out and I honestly got tired of going without a vehicle for days at a time while they tried chasing the problem.

After each part was changed no codes would pop up for at least a week or so. Once I started to think I was golden, boom, codes again.

Once I changed out the TPS I got a P0420. Reset it for it to come back a few days later. Coincidentally I changed the oil and filter after the second 420 code and for whatever reason I didn't get any codes of any kind for well over a month.

Then suddenly a few days ago the dreaded 420 came back, followed shortly by the 300, 303 & 304. And they've been tripping daily. Usually after I've been driving for over 20 minutes or so on the highway.

Before I changed out the TPS I would occasionally get the 300 codes and the engine would get thrown into "limp mode". So far that hasn't happened. The engine seems to be running just fine despite the codes and I don't appear to be losing any gas mileage so running it this way isn't a big deal so far, but I'm getting annoyed with the CEL light.

I'm contemplating changing out the O2 sensors (they're probably original) but I'm at a loss as to what it could be now. I'm hoping it's not the CAT as I don't want to spend a truckload of money to fix something only for the misfire codes to comeback anyway.

Any ideas guys?

As an aside, could a dent in the exhaust pipe restrict the flow enough to cause a misfire as wells as setting off the O2 sensor?

my guess is the wiring harness. If you have a 4.0 there is a problem area at the rear passenger side of the valve cover where the harness rubs on one of the studs. Look there, repair as needed. Should take care of your misfires if there is damage there.

my guess is the wiring harness. If you have a 4.0 there is a problem area at the rear passenger side of the valve cover where the harness rubs on one of the studs. Look there, repair as needed. Should take care of your misfires if there is damage there.

I'm thinking electrical issue too.
In my experience most dealerships don't do a great job tracking down wiring issues, they just throw parts at it.

Also, there was a TSB about the heat from the engine affecting the fuel rail, which in turn would throw codes when the injectors would act up. The fix was that they would install a heat shield around the fuel rail to keep the heat off it.

There was yet another TSB about a dirty throttle body which could also cause the same symptoms. The fix for that one was to use Mopar Combustion Chamber Cleaner and spray it into the TB until the engine idled funny...then keep spraying the entire can in there. Then shut the engine off and let it soak for a couple hours. Supposedly that cleared up a lot of the misfire codes on the dash.

I chased all these codes myself last month, but mine actually ended up being a mechanical problem with the head itself. I had to get another head machined and then installed that and fixed my issue.

my guess is the wiring harness. If you have a 4.0 there is a problem area at the rear passenger side of the valve cover where the harness rubs on one of the studs. Look there, repair as needed. Should take care of your misfires if there is damage there.

Also, there was a TSB about the heat from the engine affecting the fuel rail, which in turn would throw codes when the injectors would act up. The fix was that they would install a heat shield around the fuel rail to keep the heat off it.

There was yet another TSB about a dirty throttle body which could also cause the same symptoms. The fix for that one was to use Mopar Combustion Chamber Cleaner and spray it into the TB until the engine idled funny...then keep spraying the entire can in there. Then shut the engine off and let it soak for a couple hours. Supposedly that cleared up a lot of the misfire codes on the dash.

I chased all these codes myself last month, but mine actually ended up being a mechanical problem with the head itself. I had to get another head machined and then installed that and fixed my issue.

I was wondering about this. Sometimes the engine compartment gets so hot that I cannot even lean on the fenders from the heat transfer. I was seriously wondering if it was a thermostat issue causing the engine to overheat enough to cause the misfiring. Although the temp gauge stays at a steady 210 or so and doesn't fluctuate as far as I can tell, that might be part of the problem.

I'll stop by a store and pick up some cleaner for the throttle body. When I changed out the TPS it did look a bit dirty, but at the time I didn't have time to clean it and since the codes didn't come back for a while I forgot all about it.

my guess is the wiring harness. If you have a 4.0 there is a problem area at the rear passenger side of the valve cover where the harness rubs on one of the studs. Look there, repair as needed. Should take care of your misfires if there is damage there.

I'm hoping that it was really this easy, but time will tell.

Left work today and checked the wiring around the engine only to find that a connection was actually sitting up against the rear corner of the engine block (passenger side). So I moved the connection away from the block as far as I could, reset the CEL and then drove home.

No codes. CEL didn't even flash once.

Perhaps the heat from the engine was screwing up the conductivity in the connector as it got hotter from driving which eventually caused it to throw some codes.

I'm not going to get my hopes up but I'll figure out a way to keep the connector from touching the engine permanently and drive it around to see it it codes again.

Im unclear as to any diagnostics done. Have you swapped coil packs between bad and good cylinders to rule those out? Swapping fuel i jectors isnt horrible on this motor.
Misfires can easily destroy a cat-which would bring up a 420. Can you monitor the misifre pids on your scan tool? I think you need to step back to the basics and address an actual misfire before going anywhere else.

You didn't look where I suggested. the wiring harness issue at the back of the valve cover is very often responsible for this type of problem. Follow the coil rail back to it's connector and keep going about 6", look there for damage to wires, you will have to unwrap them.
Duh- it's a 4.0, can't swap coil packs, it's all in one rail. Otherwise you are correct, checking codes is preliminary to solving problems.

You didn't look where I suggested. the wiring harness issue at the back of the valve cover is very often responsible for this type of problem. Follow the coil rail back to it's connector and keep going about 6", look there for damage to wires, you will have to unwrap them. Duh- it's a 4.0, can't swap coil packs, it's all in one rail. Otherwise you are correct, checking codes is preliminary to solving problems.

My tj had wires and a distributor, but didnt the newer 4.0 switch to coil packs? I could be wrong on my memory. I dont know on his specific year.

You didn't look where I suggested. the wiring harness issue at the back of the valve cover is very often responsible for this type of problem. Follow the coil rail back to it's connector and keep going about 6", look there for damage to wires, you will have to unwrap them.
Duh- it's a 4.0, can't swap coil packs, it's all in one rail. Otherwise you are correct, checking codes is preliminary to solving problems.

I did look, I just haven't had a chance to unwrap them yet to do a full inspection. I noticed the connection sitting against the block and moved it aside and had no codes driving home. So I figured I'd give it a try to see if that was part of the problem, but did have codes driving to work, and back and forth since then. No 0420 code since moving the connector, just 0300 so far.